Ionization-triggered low exciton binding energy in covalent organic frameworks for efficient photocatalytic synthesis of benzimidazole

Abstract

In the field of photocatalytic organic transformations, the spotlight is shining on covalent organic frameworks (COFs), the widespread utilization of which, however, is plagued by their high exciton dissociation energy (E_b). To address this issue, herein a facile strategy was developed to synthesize an ionic COF (NQ-COF_D4-Me) with robust stability by grafting methyl groups onto the N ends of non-substituted quinoline (NQ) linkages. Ionization induced a strong polarization effect, giving rise to broad absorption in the visible light region and reduced E_b (78 meV versus 89 meV). With efficient generation, spatial separation, and transport of photogenerated charge carriers, the production of ˙O₂⁻ was accelerated over NQ-COF_D4-Me. As a result, when applied in ˙O₂⁻ mediated photocatalytic benzimidazole synthesis, NQ-COF_D4-Me achieved high yields of target products (91–98%), much better than those of the charge neutral counterpart NQ-COF_D4. In addition, the wide substrate adaptability scope and excellent recyclability of NQ-COF_D4-Me were also attested.